[en] In this work, we report a combined experimental and theoretical study on molecular structure and spectroscopic properties of the most stable conformers of PCA. ¹H, ¹³C NMR and 2D COSY NMR, ESR, IR and electronic spectroscopies were coupled with DFT theoretical calculations performed at the B3LYP/6-31G** level. The calculated geometrical parameters for the neutral protocatechuic acid PCA-H₃, its anions, its oxidized forms and the peroxo-derivative [PCA-H-O₂]^2- are in line with the experimental data. The neutral catecholate is the most stable form of PCA-H₃ whilst the dianion [PCA-H]^2- presents higher energy. This anion is (experimentally) stable only under argon, reacting with dioxygen, in the presence of air. The semiquinone [PCA-H-sq(3)]^- is very close in energy from [PCA-H-sq(4)]^- form and an equilibrium between these two oxidized radical forms might be expected. The energetically advantageous pathway for preparation of the symmetrically delocalized [PCA-sq]^2- is to oxidize the [PCA]^3-. The occurrence of this radical dianion form was justified experimentally by ESR, IR, UV-vis and NMR spectra. The structural calculations for [PCA-H-O₂]^2- indicate that C₃ (and to a lesser extent C1) may undergo a nucleophilic attack from the 'co-ordinated' peroxo-group. The conditions for the non-enzymatic degradation of PCA have been established and some new products are observed: ionization of PCA-H₃, the presence of O₂ and aprotic solvents provide the semiquinone-superoxo adduct which is then degraded to lactones, while in protic solvents, addition of H₂O₂ and the presence of air, are essential, providing aliphatic degradation products